Method for FinFET SRAM ratio tuning

ABSTRACT

A semiconductor device and method of forming the same include a substrate having a plurality of memory cells formed thereon. A memory cell includes pass-gate transistors, pull-up transistors, and pull-down transistors. The pass-gate transistors and a portion of the pull-down transistors have different doping concentrations.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Chinese patent applicationNo. 201310471092.7, entitled “A METHOD FOR FINFET SRAM RATIO TUNING”,filed with the State Intellectual Property Office of People's Republicof China on Oct. 10, 2013, the content of which is incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to semiconductor memory devices, and moreparticularly to a semiconductor device, an electronic device includingSRAM tuned ratios, and manufacturing methods thereof.

BACKGROUND OF THE INVENTION

The ongoing development of semiconductor memory technology is drivingmemory devices to higher level of integration, lower power consumption,and faster speed. Compared to dynamic random access memory (DRAM)devices, static random access memory (SRAM) devices do not need toperiodically refresh the memory cells to maintain the data storedtherein. Therefore, SRAM devices have been widely used in manyapplications.

Recently, fin-type transistors, such as fin field effect transistors(finFETs), have been used to replace planar transistors in logicdevices. Embedded static random access memory (SRAM) devices have beenwidely used, SRAM bit cells have thus, been designed using FinFETs. In aFinFET bit cell, the channel has raised, fin-like structure, allowingthe gate to be placed on two or more sides of the channel to improveconduction and leakage control. As the fin width is constant, it is notconvenient to use the width of the active area to define the finFET SRAMratio as is the case in conventional planar SRAM devices.

When the SRAM bit cells are formed using finFETs, the effective width isconstant, the SRAM ratio may only be adjusted by changing the criticaldimension (CD). However, when compared with planar SRAM devices, finFETRAM devices may have a smaller process window. It is thus challenging towiden the process window for finFET formed SRAM devices.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide a method of forming asemiconductor device. The method includes providing a front-end devicehaving a plurality of memory cells. Each memory cell includes at leasttwo pull-up transistors, at least two pull-down transistors, and atleast two pass-gate transistors. The pull-up transistors, the pull-downtransistors, and the two pass-gate transistors each have a gate andopposite gate sidewalls. The method further includes performing a firstlightly doped source and drain ion implantation into the at least twopull-down transistors and the at least two pass-gate transistors, andperforming a second lightly doped source and drain ion implantation intoa portion of the at least two pull-down transistors and the at least twopass-gate transistors.

In an embodiment, the first lightly doped source and drain ionimplantation is performed by forming a first mask over the pull-uptransistors and applying a first angled ion implantation at a first tiltangle into the pull-down transistors and the pass-gate transistors. Thesecond lightly doped source and drain ion implantation is performed byapplying a second angled ion implantation at a second tilt angle into aportion of the pull-down transistors. The first and second tilt anglesare angles relative to a direction perpendicular to a surface of thefront-end device. The first tilt angle is smaller than the second tiltangle.

In an embodiment, a portion of the pull-down transistors disposed in theshadow region of the first mask is not doped in the second angled ionimplantation.

In an embodiment, the first mask is formed by patterning a photoresistlayer.

In an embodiment, the first lightly-doped ion implantation includesforming a second mask over the at least two pull-up transistors, andperforming a lightly-doped ion implantation into the at least twopull-down transistors and the at least pass-gate transistors. Performingthe second lightly doped source and drain ion implantation includesremoving the second mask, forming a third mask over the at least pull-uptransistors and a portion of the pull-down transistors, and performing alightly doped ion implantation into transistors not being covered by thethird mask.

In an embodiment, the second and third masks are formed by patterning aphotoresist layer.

In an embodiment, the memory cell includes two pull-up transistors, fourpull-down transistors, and two pass-gate transistors.

In an embodiment, the method further includes, prior to performing thefirst lightly-doped source and drain ion implantation, applying alightly-doped ion implantation into the at least two pull-uptransistors, the at least two pull-down transistors, and the at leasttwo pass-gate transistors.

In an embodiment, the pull-up transistors, the pull-down transistors,and the pass-gate transistors have a fin-shaped structure.

Embodiments of the present invention also provide a semiconductor devicethat includes a multitude of memory cells. Each of the memory cellsincludes at least two pull-up transistors, at least two pull-downtransistors, and at least two pass-gate transistors. A portion of thetwo pull-down transistors and the pass-gate transistors have differentdoping concentrations.

In an embodiment, each of the memory cell includes two pull-uptransistors, four pull-down transistors, and two pass-gate transistors,two of the four pull-down transistors having a doping concentrationdifferent from a doping concentration of the other two pull-downtransistors.

In an embodiment, the pull-up, pull-down, and pass-gate transistors arefinFETs. In an embodiment, the semiconductor device includes a staticrandom access memory device.

The following detailed description together with the accompanyingdrawings will provide a better understanding of the nature andadvantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method of fabricating a semiconductor deviceaccording to an embodiment of the present invention;

FIGS. 2A through 2D are simplified schematic diagrams illustratingintermediate process steps of a method of fabricating a semiconductordevice according to an embodiment of the present invention;

FIGS. 3A through 3E are simplified schematic diagrams illustratingintermediate process steps of a method of fabricating a semiconductordevice according to another embodiment of the present invention;

FIG. 4 is a flow chart of a method of fabricating a semiconductor deviceaccording to an embodiment of the present invention; and

FIG. 5 is a flow chart of a method of fabricating a semiconductor deviceaccording to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings. The inventionmay, however, be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. The features may not be drawn to scale, some detailsmay be exaggerated relative to other elements for clarity. Like numbersrefer to like elements throughout.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “lateral” or “vertical” may be used herein to describe arelationship of one element, layer or region to another element, layeror region as illustrated in the figures. It will be understood thatthese terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising”, “includes”, and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

A highly doped source and drain ion implantation is referred to as anion implantation of source and drain regions with impurities to aconcentration of at least 10E18 (10¹⁸) atoms per cm³ or a dose of atleast 10E14 (10¹⁴) atoms per cm². A lightly doped source and drain ionimplantation is referred to as an ion implantation of source and drainregions with impurities to a concentration in the range of 10E14 (10¹⁴)to 10E15 (10¹⁵) atoms per cm³ or a dose of 10E10 to 10E11 (10¹⁰ to 10¹¹)atoms per cm².

The term “ratio” is referred to as the driving strength ratio betweenthe different types of transistors. For example, an “α” (alfa) ratio isreferred to as the driving strength ratio between the pull-downtransistor and the pull-up transistor. A “β” (beta) ratio is referred toas the driving strength ratio between the pull-down transistor and thepass-gate transistor. A “γ” (gamma) ratio is referred to as the drivingstrength ratio between the pull-down transistor and the pass-gatetransistor ratio.

A front-end device is referred to as a device made with front end of theline processes. A front-end device may include steps performed on awafer up to but not including formation of metal lines. A front-enddevice may include various active and passive devices such astransistors, resistors, capacitors, etc.

Embodiments of the invention are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be enlargedrelative to other layers and regions for clarity. Additionally,variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, embodiments of the invention should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result, for example, frommanufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a discretechange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the actualshape of a region of a device and are not intended to limit the scope ofthe invention.

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein.

First Embodiment

According to a first embodiment of the present invention, a staticrandom access memory (SRAM) device may include a multitude of memorycells. Each memory cell may include at least two pull-down (PD)transistors, at least two pull-up (PU) transistors, and at least twopass-gate (PG) transistors.

Processing steps of the present invention for making a semiconductordevice can be used in making logic devices other than SRAM devices. TheSRAM devices can be formed using finFETs.

FIG. 1 is a flow chart of a method A10 of fabricating a semiconductordevice according to an embodiment of the present invention. Method A10may include:

At block A1: provide a front-end device having a multitude of memorycells. A memory cell may include at least two pull-up (PU) transistors,at least two pull-down (PD) transistors, and at least two pass-gate (PG)transistors.

At block A2: perform a first lightly doped source and drain ionimplantation into the at least two PD transistors and the at least twoPG transistors.

At block A3: perform a second lightly doped source and drain ionimplantation into a portion of the at least two PD transistors and theat least two PG transistors.

In an embodiment, method A10 may include block A12 (not shown) betweenblocks A1 and A2. Block A12 may include applying a third ionimplantation to implant third lightly doped source and drain regions ofthe at least two PU transistors, the at least two PD transistors, andthe at least two PD transistors.

In a specific embodiment, in block A1, the PU, PD, and PG transistorseach have a fin-typed structure, so that the PU, PD, and PG transistorsare finFET-type transistors. The semiconductor device may include SRAMmemory cells or other memory devices formed using the finFET-typed PU,PD, and PG transistors.

In this embodiment, the first and second ion implantation to implantfirst and second lightly doped source and drain regions in respectiveblocks A2 and A3 are not limited to specific ion implantations.

In this embodiment, the method mainly includes performing lightly dopedion implantation into PD and PG transistors. The method may includeperforming lightly doped ion implantation (LDD) into logic devices(including PU transistors, PD transistors, and PG transistors). Thisstep may be omitted. After performing ion implantation into the PGtransistors and PD transistors of the memory cells, the method includesperforming a second lightly doped ion implantation into a portion of thePD transistors (the lightly doped ion implantation may be concurrentlyperformed into the PG transistor) to form PD and PG transistors havingdifferent dopant concentrations. When the semiconductor device is aSRAM, the SRAM device can have different ratios. This method isparticularly suitable to solve the small process window issue offinFET-typed SRAM devices having a constant width.

In the following exemplary embodiment, the semiconductor device includesa SRAM device. The SRAM device may include a memory cell having 4 PDtransistors, 2 PU transistors, and 2 PG transistors.

Referring to FIGS. 2A through 2D and FIG. 4, a method for manufacturinga finFET-type memory device may include:

At block B1: provide a front-end device including a multitude of memorycells. FIG. 2A is a plan view of a memory cell according to anembodiment of the present invention. The memory cell includes 2 PUtransistors PU-1 and PU-2, 4 PD transistors PD-1, PD-2, PD-3, and PD-4,and 2 PG transistors PG-1 and PG-2, as shown in FIG. 2A.

In an embodiment, the PU transistors, the PD transistors, and the PGtransistors each include a fin structure, a gate, and gate sidewalls(not shown). FIG. 2A is a plan view of a memory cell of a front-enddevice provided at block B1 of FIG. 4.

At block B2: a first mask 101 is formed on the two PU transistors PU-1and PU-2, as shown in FIG. 2B. Thereafter, a first lightly doped ionimplantation 211 at a first tilt angle “a” is performed into the 4 PDtransistors PD-1, PD-2, PD-3, and PD-4 and the 2 PG transistors PG-1 andPG-2, as shown in FIG. 2C.

FIG. 2B is a plan view of the memory cell after the first mask 101 hasbeen formed. FIG. 2C is a simplified cross-sectional view of FIG. 2Btaken along the line AA. The arrows in FIG. 2C represent the directionof ion implantation into the front-end device.

The tilt angle “a” is an angle relative to the perpendicular directionof the lateral surface of the front-end device, as shown in FIG. 2C. Inan embodiment, the angle “a” is small and can be 0 degree to ensure thatareas not masked by the mask 101 are doped by the first lightly dopedion implantation.

At block B3: a second lightly doped ion implantation 212 at a secondtilt angle “b” is performed into a portion of the 4 PD transistors PD-1,PD-2, PD-3, and PD-4 of the front-end device. In a specific embodiment,because PD transistors PD-1 and PD-4 are disposed in the shadow regionof the mask 101, only PD transistors PD-2 and PD-3 may be doped by thesecond ion implantation 212.

The second angle “b” is an angle relative to the perpendicular directionof the lateral surface of the front-end device, as shown in FIG. 2D. Thesecond angle “b” is relatively large so that mainly the PD transistorsPD-2 and PD-3 are doped by the second ion implantation. The first angle“a” is smaller than the second angle “b”, as shown in FIGS. 2C and 2D.

At block B3, PD transistors PD-1 and PD-4 are disposed near the firstmask 101 and may not be ion implanted in the second ion implantation212, as shown in FIG. 2D.

Obviously, the two PG transistors PG-1 and PG-2 are disposed at asimilar distance from the first mask 101 as those of PD transistors PD-2and PD-3, they will also be doped by the second ion implantation 212.However, because the PG transistors PG-1 and PG-2 are disposed away fromthe PD transistors PD-2 and PD-3, PG-1 and PG-2 may be doped with a doseor doping concentration relatively lower than the dose or dopingconcentration of PD-2 and PD-3.

In an embodiment, the first mask 101 may be formed by patterned aphotoresist layer or any suitable mask materials.

In an embodiment, the method may include an intermediate process blockB12 (not shown) between process blocks B1 and B2. Block B12 may includeperforming a lightly doped ion implantation into the two PU transistors,the four PD transistors, and the two PG transistors.

This method of manufacturing a semiconductor device provides pass-gatetransistors and pull-down transistors having different dopantconcentrations to obtain different SRAM device ratios.

Second Embodiment

FIGS. 3A through 3E and FIG. 5 illustrate an exemplary implementation ofa method C10 for fabricating a semiconductor device according to asecond exemplary embodiment of the present invention. FIGS. 3A through3E illustrate intermediate process stages of a memory cell, and FIG. 5illustrates a flow chart of a method for fabricating the memory cell, inaccordance with the second exemplary embodiment of the presentinvention. Method C10 may include:

At block C1: a front-end device having a multitude of memory cells isprovided. FIG. 3A is a plan view of a memory cell according to theexemplary embodiment of the present invention. The memory cell includestwo PU transistors PU-1 and PU-2, four PD transistors PD-1, PD-2, PD-3,and PD-4, and two PG transistors PG-1 and PG-2, as shown in FIG. 3A.

The PU, PD, and PG transistors each include a fin structure, a gate, andgate sidewalls (not shown).

At block C2: a mask 102 is formed on the two PU transistors PU-1 andPU-2, as shown in FIG. 3B. Thereafter, a first lightly doped ionimplantation 311 is performed into the four PD transistors PD-1, PD-2,PD-3, and PD-4 and the two PG transistors PG-1 and PG-2, as shown inFIG. 3C. FIG. 3B is a plan view of the front-end device after theformation of the mask 102. FIG. 3C is a cross-sectional view taken alongthe line AA of FIG. 3B. The arrows of FIG. 3C represent the direction ofthe lightly doped ion implantation 311. The lightly doped ionimplantation 311 may be performed at a tilt angle similar to the tiltangle “a” of FIG. 3C (2C in Chinese specification) to ensure that areasnot covered by the mask 102 are lightly doped.

At block C3: the mask 102 is removed. A mask 103 is then formed on thetwo PU transistors PU-1 and PU-2 and a portion of the PD transistors,e.g., PD-1 and PD-4 that are near the respective PU-1 and PU-2.Thereafter, a second lightly doped ion implantation 312 is performed onthe PD transistors PD-2 and PD-3 that are not masked by the mask 103, asshown in FIG. 3E.

Of course, the PG transistors PG-1 and PG-2 that are not masked by themask 103 are also lightly doped by the second lightly doped ionimplantation 312. However, because of their position relative to thePD-2 and PD-3, the PG transistors PG-1 and PG-2 may have a dose lowerthan the dose of the PD transistors PD-2 and PD-3.

FIG. 3D is a plan view of the front-end device after the formation ofthe mask 103. FIG. 3E is a cross-sectional view taken along the line AAof FIG. 3D. The arrows shown in FIG. 3E represent the direction of thesecond lightly doped ion implantation 312. The second lightly doped ionimplantation 312 is performed at a tilt angle similar to the tilt angleat block C2. In other words, the tilt angle of the second lightly dopedion implantation 312 may be in the same range as the tilt angle “a” ofFIG. 2C described above. The tilt angle of second lightly doped ionimplantation 312 is small to ensure that areas not masked by the mask103 (i.e., PD-2 and PD-3) are doped by the second lightly doped ionimplantation 312.

Because of the mask 103, this approach allows a more controllable andprecise lightly doped ion implantation comparing to the process in stepB3 described above that uses the shadow effect of the mask 101 and arelative large tilt angle to prevent that PD-1 and PD-4 from being dopedby the first lightly doped ion implantation. Indeed, the blocks C1 andC2 of the second embodiment are the same as blocks B1 and B2 of thefirst embodiment, the difference is in block C3 (and B3) where the mask103 is added, so that the lightly doped source and drain ionimplantation (i.e., the second lightly doped ion implantation) can bemore accurate and controllable.

In an embodiment, the mask 102 and the mask 103 may be a patternedphotoresist layer or any suitable materials.

In an embodiment, there may be an intermediate process step C12 betweenthe blocks C1 and C2. The intermediate process C12 may include applyingan lightly doped source and drain ion implantation to the two PUtransistors, four PD transistors and two PG transistors of the memorycell. This embodiment provides a method of manufacturing a semiconductordevice having transfer gate transistors and pull-down transistors thathave different concentrations to obtain SRAM devices with differentratios.

It is noted that there may be other process steps after the blocks A3,B3, or C3 for the completion of the semiconductor device. In the examplein FIGS. 3A-3E, a memory cell having 4 PD transistors, 2 PU transistors,and 2 PG transistors has been described, but it is understood that theinvention is not limited thereto. For example, embodiments of thepresent invention are applicable to memory cells each having 2 or morePD transistors, 2 or more PU transistors, and 2 or more PG transistors.It is also understood that the present invention is not limited to SRAMdevices and is applicable to all the above memory cell structures andother semiconductor devices.

According to embodiments of the present invention, by performing lightlyion source and drain implantation with different concentrations to thetransistors, pass-gate transistors and pull-down transistors withdifferent doping concentration are formed. When the semiconductor deviceis a SRAM device, the SRAM device may have different ratios.

FIG. 1 is a flow chart of a method A10 for manufacturing a semiconductordevice according to an embodiment of the present invention. Method A10includes:

At block A1: provide a front-end device including a multitude of memorycells. Each memory cell includes at least two pull-up transistors, atleast two pull-down transistors, and at least two pass-gate transistors.The pull-up, pull-down, and pass-gate transistor each have a gate andgate sidewalls;

At block A2: perform a first lightly-doped source and drain ionimplantation into the at least two pull-up transistors, at least twopull-down transistors, and at least two pass-gate transistors;

At block A3: perform a second lightly-doped source and drain ionimplantation into a portion of the at least two pull-down transistors.

FIG. 4 is a flow chart of a method B10 for manufacturing a semiconductordevice according to another embodiment of the present invention. MethodB10 includes:

At block B1: provide a front-end device including a multitude of memorycells. Each memory cell includes two pull-up transistors, four pull-downtransistors, and two pass-gate transistors;

At block B2: form a first mask over the two pull-up transistors; andperform a first lightly-doped source and drain ion implantation at afirst tilt angle into the four pull-down transistors, and the twopass-gate transistors of each memory cell;

At block B3: perform a second lightly-doped source and drain ionimplantation at a second tilt angle into a portion of the four twopull-down transistors. The first and second tilt angles are ionimplantation angles in the direction perpendicular to the upper surfaceof the front-end device. The first tilt angle is smaller than the secondtilt angle.

FIG. 5 is a flow chart of a method C10 for manufacturing a semiconductordevice according to another embodiment of the present invention. MethodC10 includes:

At block C1: provide a front-end device including a multitude of memorycells. Each memory cell includes two pull-up transistors, four pull-downtransistors, and two pass-gate transistors;

At block C2: form a first mask over the two pull-up transistors; andperform a first lightly-doped source and drain ion implantation at afirst tilt angle into the four pull-down transistors, and the twopass-gate transistors of each memory cell;

At block C3: remove the first mask; form a second mask over the twopull-up transistors and a portion of the pull-down transistors; andperform a second lightly-doped source and drain ion implantation at asecond tilt angle into the portion of the pull-down transistors. Thefirst and second tilt angles are ion implantation angles in thedirection perpendicular to the upper surface of the front-end device. Inan embodiment, the first and second tilt angles may be the same.

In other embodiment, a semiconductor device includes a multitude ofmemory cells. Each memory cell may include at least two pull-uptransistors, at least two pull-down transistors, and at least twopass-gate transistors. The at least two pull-up transistors and aportion of the pull-down transistors and the pass-gate transistor aredoped with different doping concentrations.

In yet another embodiment, a semiconductor device includes a multitudeof memory cells. Each memory cell may include two pull-up transistors,four pull-down transistors, and two pass-gate transistors. Two of thefour pull-up transistors have different doping concentrations than theother two pull-up transistors.

In an embodiment, the pull-up, pull-down, and pass-gate transistor arefinFET-typed transistors. The semiconductor device is a static randomaccess memory device. By ion implanting different doping concentrationsto the pass-gate transistors and the pull-down transistors, the SRAMdevice may have different ratios.

Embodiments of the present invention also provide an electronic devicethat may include a semiconductor device having memory cells of thestructures of the different exemplary embodiments described above, suchas memory cells having different numbers of pull-up, pull-down, andpass-gate transistors described above. Because the electronic deviceuses such semiconductor devices having different ratios, the electronicdevice may have a better performance. In an embodiment, the electronicdevice can be a mobile phone, tablet PC, laptop, netbook, game console,still camera, video camera, voice recorder, MP3, MP4, PSP, and the like.

It is to be understood that the above described embodiments are intendedto be illustrative and not restrictive. Many embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinednot with reference to the above description, but instead should bedetermined with reference to the appended claims along with their fullscope of equivalents.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: providing a front-end device having a plurality ofmemory cells, each memory cell including at least two pull-uptransistors, at least two pull-down transistors, and at least twopass-gate transistors, the pull-up transistors, the pull-downtransistors, and the two pass-gate transistors each having a gate andgate sidewalls; forming a mask over the at least two pull-uptransistors; performing a first lightly doped source and drain ionimplantation at a first tilted angle into the at least two pull-downtransistors and the at least two pass-gate transistors; and performing asecond lightly doped source and drain ion implantation at a secondtilted angle into a portion of the at least two pull-down transistorsand the at least two pass-gate transistors; wherein the first tilt angleand the second tilt angle are angles relative to a directionperpendicular to a surface of the front-end device, and the first tiltangle is smaller than the second tilt angle.
 2. The method of claim 1,wherein performing the second angled ion implantation at a second anglecomprises: using a shadow region of the mask to prevent a portion of thepull-down transistors disposed within the shadow region from beingdoped.
 3. The method of claim 1, wherein forming the first maskcomprises: patterning a photoresist layer.
 4. The method of claim 1,wherein the memory cell comprises two pull-up transistors, fourpull-down transistors, and two pass-gate transistors, the second ionimplantation is performed on two pull-down transistors.
 5. The method ofclaim 1, further comprising, prior to performing the first lightly dopedion implantation: performing a lightly-doped source and drain ionimplantation into the at least two pull-down transistors and the atleast two pass-gate transistors.
 6. The method of claim 1, wherein thepull-up transistors, the pull-down transistors, and the pass-gatetransistors are fin-FET transistors.
 7. The method of claim 1, whereinthe semiconductor device is a static random access memory.
 8. A methodof forming a semiconductor device, the method comprising: providing afront-end device having a plurality of memory cells, each memory cellincluding at least two pull-up transistors, at least two pull-downtransistors, and at least two pass-gate transistors, the pull-uptransistors, the pull-down transistors, and the two pass-gatetransistors each having a gate and gate sidewalls; forming a first maskover the at least two pull-up transistors; and performing alightly-doped ion implantation into the at least two pull-downtransistors and the at least pass-gate transistors; removing the firstmask; forming a second mask over the at least two pull-up transistorsand a portion of the pull-down transistors; performing a lightly dopedion implantation into transistors not being covered by the second mask.9. The method of claim 8, wherein forming the first mask and the secondmask comprises: patterning a photoresist layer.
 10. The method of claim8, wherein the pull-up transistors, the pull-down transistors, and thepass-gate transistors are fin-FET transistors.
 11. A method of forming asemiconductor device, the method comprising: providing a front-enddevice having a plurality of memory cells, each memory cell includingtwo pull-up transistors, first, second, third, and fourth pull-downtransistors arranged sequentially between the two pull-up transistors,and two pass-gate transistors; forming a first mask over the two pull-uptransistors; performing a first lightly doped source and drain ionimplantation into the first, second, third, and fourth pull-downtransistors and the two pass-gate transistors; removing the first mask;forming a second mask over the two pull-up transistors and the first andfourth pull-down transistors in a vicinity of the two pull-uptransistors; and performing a second lightly doped source and drain ionimplantation into the third and fourth pull-down transistors not coveredby the second mask and the two pass-gate transistors.
 12. The method ofclaim 11, wherein the pull-up transistors, the pull-down transistors,and the pass-gate transistors are fin-FET transistors.